Speech and Language

Lecture for the 2nd BM course

Dr Jan Schnupp

jan.schnupp@dpag.ox.ac.uk

What the syllabus says you should know (§20.4.3-20.4.5)

• Core material: – Mechanisms of speech production– Aphasias: Wernicke’s area and sensory

aphasia; Broca’s area and motor aphasia. – Hemispheric specialization

• Extension Material:– Speech sounds and speech perception – Arcuate bundle. The dyslexias

Part 1: Producing Speech

Vocal Folds in Action

http://mustelid.physiol.ox.ac.uk/drupal/?q=vocal_folds

Articulation

• Articulators (lips, tongue, jaw, soft palate) move to change resonance properties of the vocal tract.

• https://mustelid.physiol.ox.ac.uk/drupal/?q=vocalization/articulators

Harmonics and Formants of Speech Sounds

FormantHarmonic

Can other animals speak?

• Other mammals have similar vocal tracts and use them for communication. However, they have only very limited use of syntax (grammar) and very much smaller vocabularies than humans.

• http://mustelid.physiol.ox.ac.uk/drupal/?q=mishka

Broca’s (Motor or Expressive) Aphasia

• Patients with Broca’s Aphasia tend to understand speech well.

• However, they have great difficulty articulating speech themselves, even though there is no severe paralysis of the articulators in the vocal tract.

• Their speech tends to be halting, laboured, consisting of isolated utterances rather than full, grammatical sentences.

• Suggests underlying cause is a difficulty in planning highly complicated motor acts.

A Patient with Broca’s Aphasia

• From the archives of the University of Wisconsin

• http://mustelid.physiol.ox.ac.uk/drupal/?q=brocas_aphasia

Broca’s Area

• Broca’s aphasia is usually associated with lesion to the left frontal cortex.

• See here the brain of Broca’s Patient, Mr Leborgne (“TanTan”) features a large lesion in Broca’s area.

Part 2 – Hearing Speech

Speech as a “Dynamic” Acoustic Signal

• Most of the information in speech is carried in the manner in which the formant structure varies over time.

• Pitch carries little or no semantic information (but is important in prosody and in “pitch tracking” for speech in noise.

• Think of the role of the tonotopic axis in the ascending auditory pathway as representing formant distributions. (Not pitch!)

Speech and Cochlear Implants

• To make speech comprehensible, cochlear implants reproduce the formant structure as a pattern of electrical stimulation along the tonotopic axis of the Basilar Membrane.

• https://mustelid.physiol.ox.ac.uk/drupal/?q=prosthetics/

noise_vocoded_speech

• Spectrogram (B) and Auditory (VIII) Nerve “Neurogram” (A) of a spoken sentence.

• From Delgutte B (1997) Auditory neural processing of speech. In: The Handbook of Phonetic Sciences (Laver WH, ed), pp 507-538. Oxford: Blackwell.

Auditory Cortex Neurograms of American English Onset Consonants (Engineer et al. Nat Neurosci 2008)

Where in the Brain does the Transition from Sound to

Meaning happen?• We don’t really know. • “Ventral vs Dorsal stream hypothesis”

of auditory cortex connectivity would suggest that anterior temporal and frontal structures should be involved.

• This fits with neuroimaging studies (e.g. Scott et al (2000) Brain 123 Pt 12:2400-2406)

• https://mustelid.physiol.ox.ac.uk/drupal/?q=node/46

• But other electrophysiological and lesion data do not really fit this picture.

Part 3

More about Aphasias and Clinical Observations

Receptive (Wernicke’s Aphasia)

• Patients with Wernicke’s Aphasia usually have great difficulty understanding speech, even though there is no deafness.

• They may speak fluently, often in long sentences, but the meaning of their sentences is unclear. (“fluent paraphasia”).

A Patient with Wernicke’s Aphasia

• From the archives of the University of Wisconsin

Wernicke’s Area

• Wernicke’s aphasias are often associated with lesions at the boundary of the superior temporal and parietal lobes on the left hemisphere.

Conduction Aphasia

• Patients may be able to understand speech as well as produce meaningful speech, but have difficulty repeating a spoken sentence.

• Often associated with damage to the Arcuate Fasciculus, which connects Wernicke’s area with frontal pre-motor structures.

The Arcuate Fasciculus

Big fibre bundle connecting Broca’s and Wernicke’s Areashttp://www.biocfarm.unibo.it/aunsnc/pictef14.html

Categorizing Aphasias

• Brain lesions are rarely confined to particular “classical” area boundaries.

• The symptoms used to diagnose and classify aphasias can vary considerably in severity.

• Thus, aphasic patients may not fit the diagnostic categories terribly well, and the way aphasias are categorized are themselves evolving.

Cortical Speech Areas and Neurosurgery

• Surgeons attempting to remove epileptic foci or tumours from the brain are anxious to avoid damaging areas that are crucial for speech production or comprehension.

• They may use “Wada tests” or temporary functional lesioning trough direct electrocortical stimulation.

• Further reading: Calvin & Oja “Conversations with Neil’s Brain”.

Hemispheric “Dominance” for Speech and the Wada test

• Broca first proposed that the left hemisphere is “dominant” for speech, based on examinations of post-mortem brains.

• Nowadays “dominance” is usually assessed with the “Wada test” (intracarotid sodium amobarbital procedure): either the left or right brain hemisphere is anesthetised by injection of amobarbital into the carotid through a catheter. The patient’s ability to understand and produce speech is scored.

Left Hemisphere Dominance Dominates

Wada test results suggest that:• Ca 90% of all right handed patients and ca. 75%

of all left handed patients display “left hemisphere dominance” for speech.

• The remaining patients are either “mixed dominant” (i.e. they need both hemispheres to process speech) or they have a “bilateral speech representation” (i.e. either hemisphere can support speech without necessarily requiring the other).

• Right hemisphere dominance is comparatively rare, and seen in no more than 1-2% of the population

Hierarchical levels of speech perception

• Acoustic / phonetic representation:- Can the patient tell whether two speech sounds or syllables presented in succession are the same or different?

• Phonological analysis:- Can the patient tell whether two words rhyme? Or what the first phoneme (“letter”) in a given word is?

• Semantic processing:- Can the patient understand “meaning”, e.g. follow spoken instructions?

“Functional Lesioning” by Electrocortical Stimulation

• Sites where direct electrical stimulation can disrupt acoustic/phonetic (A), phonological (B) or semantic (C) processing of speech.

• From Boatman D (2004) Cortical bases of speech perception: evidence from functional lesion studies. Cognition 92:47-65.

If you want to know more

• Try chapters 1,2, 4 and 8 of “Auditory Neuroscience” by Schnupp, Nelken & King, MIT Press.

• Check out auditoryneuroscience.com

That’s all Folks